1. Field of the Invention (Technical Field)
The present invention relates to a method and apparatus for thermal shock reliability testing of electronic components such as printed circuit boards and solder joints. The present invention provides for testing the thermal reliability of devices in a time period which is up to ten times shorter than currently used tests.
2. Background Art
Note that the following discussion is given for more complete background of the scientific principles and is not to be construed as an admission that such concepts or publications listed are prior art for patentability determination purposes.
Thermal shock testing has long been the accepted method to check the reliability of plated-through holes and solder joint connections in electronic components. The first standard was MIL-STD-202 Method 107, which originated in the late 1950's and was last updated in 1984. For printed circuit boards and solder joints, the acceleration mechanism for reliability is a function of the thermal coefficient of expansion of the materials used in the device under test (DUT). Along with the difference between the temperature extremes of the test environment, this coefficient determines the stresses introduced in the DUT and the reliability acceleration that is exhibited.
Thermal shock conditions are produced by rapidly moving the DUT between two temperature extremes, and typically require that the transition time between the extremes is less than five minutes, thereby creating a shock condition. Ordinarily the DUT has a “coupon” incorporated into it, or alternatively, a standalone test coupon is used, and the electrical characteristics, such as resistance, of the coupon are monitored. The coupon comprises standard components, such as vias, in a variety of layouts, and its connections to the test system are determined by the particular system. Thus it is actually the coupon, and not the DUT, that is tested. Because the coupon is manufactured by same process and (optionally) at the same time as the DUT, the reliability of the coupon is an excellent indication of the reliability of the DUT.
Typical values for the lower temperature extreme range from −40° C. to −65° C., while the upper temperature extreme typically ranges from 85° C. to 200° C. The time the DUT must remain at a temperature extreme before reaching equilibrium, or dwell time, can vary from a few minutes to an hour, depending on the method of producing the temperature extremes, the capacity for heat transmission, and the mass of the DUT. For massive parts over 136 kg, dwell times can reach eight hours. This time is required because typical test methods do not measure the temperature of the DUT samples directly, and so an estimate of the time required for the DUT to come to equilibrium at the desired temperature is required. Considering that the number of cycles for a complete test can range from hundreds to thousands of cycles, this equilibrium time is very significant.
Historically, the two most used methodologies for producing thermal shock environments are air-to-air and liquid-to-liquid. Air-to-air thermal shock systems utilize two separate chambers, each set to the opposite temperature extreme, and a mechanism to move the DUT between the two chambers. While these chambers are readily available, they are expensive to operate and provide a low heat exchange rate to the DUT. Dual liquid-to-liquid chambers, each controlled to the opposite temperature extreme, utilize special liquids, and a mechanism to move the DUT between the two liquids. Unlike the air-to-air chambers, this very expensive liquid provides an excellent heat exchange rate, and thus, is able to move the DUT rapidly between temperatures extremes. Since both these methods physically move the DUT, the cabling to the DUT must be capable of moving.
Reliability of the DUT is determined by monitoring the resistance of the samples during the test. As samples fail, the resistance changes, thus providing reliability data. For the above testing systems, which require transportation of the samples, it is very difficult to make electrical measurements during cycling, and the cabling is typically of lengths that do not allow for high accuracy measurements and limits the number of data points that can be monitored. The resultant infrequent monitoring also makes detection of glitch conditions, and even the actual failure point, marginal at best.
Another thermal shock system is the Interconnect Stress Testing (IST) system disclosed in U.S. Pat. Nos. 5,392,219, 5,451,885, and 5,701,667 to Birch et al. This method uses the copper circuits (both traces and vias) integrated into the DUT as resistance heating elements, and is cooled to ambient temperature with circulated air. This method has the advantage of predetermining the amount of current that raises the samples to the desired temperature. This eliminates the need for dwell time when the actual testing is carried out. There are a number of drawbacks to this technique, however. The current required to heat the samples to a desired temperature is not empirically determined; rather, it is calculated from the resistance of the sample at room temperature. In addition, as samples fail during the test, the resistance of the system changes, thus requiring the applied current to be modified in order to maintain the desired temperature. Unfortunately the appropriate new value of the current is difficult if not impossible to determine, resulting in poor control. For example, the upper temperature may exceed a required maximum temperature for a given DUT, such as that determined by the glass transition temperature Tg of certain materials used in the coupon or DUT. In addition, the method does not include the cold portion of the thermal cycle, so it can not replicate traditional thermal shock tests, both in low temperature extension and overall temperature range.
Accordingly there is a need for a thermal shock testing system that provides for exact determination of the sample temperature so the dwell time can be eliminated, temperature monitoring without the use of the electrical characteristics of the samples, continuous sample monitoring, and a temperature cycle with temperature extremes both above and below ambient temperature without requiring the samples to be moved.